ParaView and ParaView-derived applications have their own main window class
that inherits from QMainWindow. The standard Qt approach to intercepting
signals to the main window (e.g. "close application") is to reimplement
QMainWindow's event methods (e.g. closeEvent(QCloseEvent*)). This
polymorphic solution is not available to ParaView-derived applications
comprised of plugins, however. To facilitate a plugin's ability to influence
the behavior of ParaView's main window, this commit introduces
pqMainWindowEventManager, a manager that emits signals when the main window
receivesevents. These signals are connected to pqMainWindowEventBehavior, a
behavior class for performing tasks originally executed in
ParaViewMainWindow (with the exception of the splash screen).